A device and a process supplying substrates to an electrophotographically operated digital printing apparatus are well known from DE 100 23 828. With an electrophotographically operated digital printing apparatus that, for the sake of clarity, will be more closely explained, but without limitation to such a printing apparatus, substrates to be printed are supplied from one or more conveyor operating sequences to a paper path, or more generally to a conveyor operating sequence for substrate stock of any kind. The presence of several feeders or supply units makes it possible to supply substrate stock particularly of various formats, weights, materials, or the like. This is an advantage above all in the case of a digital printing apparatus because every single page to be printed is illustrated anew anyway. Therefore, it is also possible to reduce time for printing jobs where, for example, a single such printing job consists of pages of a brochure that are supplied to the printing apparatus one right after another and, subsequently, if the occasion arises, are also finished, in such a way that the front and back cover could be a heavier paper and following pages a lighter paper. In between, perhaps, even film with graphs or the like could be printed. These varying substrate stocks would be in different feeder units and in a previously selected sequence fed respectively as substrates in the conveyor operating system intermittently at a certain rate.
Thus, an initial section of a conveyor operating sequence beginning with the feeder units can consist, for example, of clamping belts propelled along a closed loop path, between which the substrates are conveyed. Afterwards, the substrates can be transferred and fed onto a conveyor belt powered to revolve and adhered there through electrostatic force. This conveyor belt is mostly a transparent plastic belt and leads through a printing unit that can, of course, exist for color printing from several printing devices. In electrophotographic printing a latent toner image per color extract is transferred to the substrates. Afterwards, the substrate is carried on to a fixer unit, in which the toner image is bonded to the substrate, in particular superficially fused and cooled down. A change of agency of conveyance in the conveyor operating sequence could take place for conveyance into and through the fixing unit. Thereafter, substrates to be printed on only one side will be conveyed further or released into a feeder. After fixing, substrates to be printed on both sides will be conveyed back over a conveyor operating sequence loop to the printing unit and reversed for further printing. The return transport and turning can occur simultaneously in that the clamp belts can be utilized again for this section of the conveyor operating sequence. They take a certain helixoid spiral course and therefore turn the substrates by 180° around the longitudinal axis in the transfer direction in which they are moving.
In particular, the conveyor belt feeding through the printing apparatus, the belt that frequently is labeled web in electrophotography, should be clogged with the least possible spacing between substrates in order to have the fastest possible flow rate, and thus ensure the greatest possible print production. On the other hand, the least possible spacing between successive substrates must be observed. This is true both for one-sided printing of the front sides of substrates, and also in the case of two-sided printing for printing the front sides and the reverse sides of substrates in face-and-reverse printing.
In order to achieve the optimal or best aligned allocation of the web possible, the web is theoretically or even through control systems separated into fields that can be designated as frames, in which a substrate, taking established formats into consideration, should be precisely placed for printing in each case. In that way, where there is a crisscrossing seam by which the ends of the web are bound, a field of the web is recessed in order to shape the web as a closed loop. For convenience, this seam is otherwise also used as a marker and recorded by a sensor, in order to be able to monitor the status of circulation of the web and to have a point of reference. For that reason, this seam may not be covered. Other markers would also come into consideration, of course, particularly those that are only marginally mounted on the web.
The elapsed time of the runback after printing of the first side of revolving substrates and the elapsed time of the web should stand in an integer relationship to one another so that, after return transport of the substrates, these frames are also encountered once again properly on the conveyor belt for transfer of the substrates for two-sided printing.
For all that, problems can still arise in that the substrate run times can be influenced by multiple parameters within the printing apparatus. As an example, weight and length of paper emerged as commanding paper-related influences. Similarly, machine specific parameters, e.g. precise length of the conveyor operation sequence, roll diameter, and motor speed contribute thereto.
This process is the reason for various problems affecting the run time of the printing apparatus (e.g. image quality, inadequate paper pitch). Especially noteworthy is the problem that arises on print work orders with mixed types of paper. Thus, for example, thick (heavy) sheets have shorter run times than thin sheets. For that reason, the pitch of two back-to-back substrates can lessen significantly during the flow through the apparatus (the quicker, thick one picks up). That leads then to an interruption of the printing operation because of inadequate distance between sheets and with that to a significant output loss for the apparatus. Similarly, substrates can possibly come to rest on the web-seam and thereby subject to image defects.
Thus the problem of presenting a process or a device of the kind mentioned is the basis for the invention, a process or a device that is capable of better supplying substrates.